1. Field of the Invention
The present invention relates to a technique of converting image data that are expressed by tone values of pixels constituting an image. More specifically the present invention pertains to a technique of converting the image data into a specific expression format based on the dot on-off state of each pixel.
2. Description of the Related Art
Image display devices that create dots on a display medium, such as a printing medium or the screen of a liquid crystal display, to express an image are widely used as output devices of diverse image devices. The image display device takes only either the dot on state or the dot off state as local expression, but enables display of an image of s continuously varying tone by adequately adjusting the dot formation density according to the tone values of an original image.
Typical techniques applied for such image display devices to determine the dot on-off state in each pixel and ensure dot creation with an adequate density corresponding to the tone values of the original image include an error diffusion method and a least mean square error method, which are mathematically equivalent to each other.
The error diffusion method diffuses and stores an error of tone expression, which arises in a pixel of interest due to creation of a dot or creation of no dot in the pixel, into peripheral non-processed pixels in the vicinity of the pixel of interest. The error diffusion method determines the dot on-off state in each non-processed pixel such as to cancel the tone errors diffused from peripheral processed pixels. The least mean square error method, on the other hand, stores the error of tone expression, which arises due to the determination of the dot on-off state, in the pixel of interest without diffusion into peripheral non-processed pixels. The least mean square error method reads the tone errors stored in peripheral processed pixels, and determines the dot on-off state in each non-processed pixel such as to cancel the tone errors. Either of these two techniques determines the dot on-off state such as to cancel the tone errors arising in the peripheral processed pixels, thereby creating dots with an adequate density corresponding to the tone values of the original image. Application of these techniques for determination of the dot on-off state thus enables high-quality images to be displayed by the image display device.
The error diffusion method determines the dot on-off state in each non-processed pixel, while diffusing the error of tone expression arising in each processed pixel into peripheral non-processed pixels. The process of diffusing and storing the tone error into the peripheral non-processed pixels undesirably extends the processing time required for determination of the dot on-off state. The least mean square error method, on the other hand, determines the dot on-off state in each non-processed pixel, while reading the errors of tone expression from peripheral processed pixels. The process of reading the tone errors from the peripheral processed pixels undesirably extends the processing time required for determination of the dot on-off state. Either of these conventional methods requires some time for determination of the dot on-off state and accordingly has difficulties in high-speed display of images.